Device for moving a beam of charged particles



Ba i-62$ 9487 J. w. DAVIS ET AL 3,466,487

DEVICE FOR MOVING A BEAM OF CHARGED PARTICLES Sept. 9,1969

Filed June 16, 1967 F IG.

R0774 7'/NG MA 5K ANNULAR DR/V/NC CON TOUREO GEAR CATHODE FOCUSEO ELECTRON BEAM F/G. 2 (TYPICAL) Vii CA THODE SH/ELD APE R TURES REC 7I4NGULAR CON TOUREO CATHODE INVENTORS JACK W. DAVIS FERNAND J. FERREIRA ATTORNE SR unuoo nu Lhuwa. W M

U.S. Cl. 313-146 4 Claims ABSTRACT OF THE DISCLOSURE A nondissipative apertured mask for shaping an electron beam produced from a cathode operating in a glow discharge is shown. Relative movement of the electron beam is obtained by moving the apertured mask relative to the electron emitting surface of the cathode.

Background of the invention In the copending application by'Iack W. Davis, entitled Contoured Cathode, Ser. No. 508,314, filed Nov. 17, 1965, issued from a continuation-in-part as U.S. Patent No. 3,430,091 and assigned to the same assignee, a contoured cathode is described wherein a beam of electrons is obtained from a cathode operating in the glow discharge. The contoured cathode emits electrons by various secondary emission processes such as that due to ions striking its surface and is therefore essentially a surface emitter. Focusing is accomplished by' contouring the surface of the cathode.

In the glow discharge mode, as opposed to an arch mode, practically all of the potential drop from the cathode-to-the anode occurs across a small cathode fall region adjacent the cathode. Since equipotentiallines near the, cathode follow the contoured surface, strong electrostatic focusing of the electrons emitted from the cathode surface may be accomplished. By contouring the surface of the cathode along a constant radius, the majority of the electrons will be focused at the center of the curvature which generally lies outside the cathode fall.

As described in the copending application, the chiciency of the contoured cathode may be substantially improved by the use of a shield surrounding those areas of the cathode from which no emission is desired. These shields may be made of a conductor material or an insulator. The shield is selectively spaced from the cathode to suppress the emission of electrons from those surfaces which would not contribute to the main beam. The contoured cathode permits the design of a shield of unusual effectiveness in inhibiting the formation of a plasma in the gap between the shield and the cathode. It is further taught in the copending application that the shield may be used to mask the frontal emission surface of the cathode to thereby prevent the ions in the glow discharge from reaching the cathode surface and thus shaping the electron beam.

The possibility of utilizing the shield to mask the electron beam provides an unusual device for moving a beam of electrons. In welding with an annular cathode, a uniform disc-like beam is formed which produces a single molten zone all around the entire circumference of the joint. In any diameter thin'wall materials such as less than V: inch pipe or tubing, such welds generally are quite excellent. The welding plane, however, in such a case is horizontal for in that position the surface tension forces act essentiallyvvertically across the weld and tend to contain the molten material. However, as soon as the weld plane is inclined to the horizontal, the molten material, except for that in the surface film, is free to United States Patent Patented Sept. 9, 1969 flow circumferentially around the weld joint to the lowest point under the action of gravity. When the pressure of the melt at this low point exceeds that which can be contained by surface tension forces, the surface film will rupture and molten material will spill out. For the welding of large pipe or rod sizes such a problem can be tremendous and the solution to such a problem is to divide the molten material into small individual zones in a manner which maintains a high ratio of surface force relative to the mass of molten material.

Summary of the invention It is therefore an object of this invention to provide a glow discharge operated cathode structure utilizing a nondissipati-ve apertured mask and moving the mask relative to the cathode for obtaining a moving electron beam.

This object is obtained by providing a cathode structure operating in a glow discharge and producing a beam of electrons which are emitted from the cathode due to the bombardment of the emitting surface by ions, photons, metastables and fast neutrals and wherein a shield having an aperture therein is moved relative to the cathode emitting surface to produce a moving beam of electrons emanating through the aperture.

Description of the preferred embodiment A first embodiment-is shown in FIGURE 1 wherein an annular contoured cathode is provided with an annular rotating beam obtained from a rotating mask or shield.

FIGURE 2 illustrates a linear contoured cathode with a slidable mask or shield provided with a plurality of apertures for obtaining a plurality of beams therefrom.

In FIGURE 1 a contoured cathode 10 is surrounded by a shield 11 which may be at anode potential or electrically floating. An anode 12 is provided at any convenient place within the chamber in which the cathode 10 is located. The chamber is evacuated to a suitable pressure for establishing the glow discharge and an electron beam from the cathode. The cathode 10 is annular and provided with a frontal surface 14 which is contoured in the axial direction. A workpiece 15 is located concentric to the annular cathode 10. A power supply 16 provides the high negative potential necessary for producing the glow discharge. An insulated connection of the power supply 16 through the shield 11 to the cathode 10 and a supporting arrangement for the cathode 10 within the shield 11 are more specifically shown and described in the copending application Ser. No. 508,201, filed Nov. 17, 1965 entitled Mounting for a Glow Discharge Cathode by Allan P. Walch and assigned to the same assignee.

Around the cathode 10 the shield 11 is spaced selectively therefrom to suppress the glow discharge in the gap between it and the cathode. The precise spacing depends on the cathode voltage, the type of gas between the cathode and shield and the gas pressure in accordance with the well-known Law of Paschen. The shield 11 has a rotating masked member 17, concentric with the annular cathode 10. The shield member 17 is rotatably mounted on the shield 11 and opposite to the electron emitting surface of the cathode 10. Shield member 17 prevents the particles from reaching the surface 14 except for an aperture 18.

The aperture 18 may have any desired shape commensurate with the type of electron beam desired. The top circumferential edge of the rotating shield member 17 is provided with teeth that mesh with a driving gear 19. The driving gear in turn is coupled to a shaft 20 and then to a motor 21. The motor is energized by a voltage source (not shown) to rotate the mask 17 relative to the emitting surface 14.

As illustrated in FIGURE 1, the shield member 17 is curved to maintain a predetermined distance from the frontal surface 14 to suppress the glow discharge everywhere but from that surface in front and opposite to the aperture 18. As a result, the particles are permitted to travel towards the frontal surface 14 and a beam of electrons from the surface towards the workpiece 15 is obtained. If thereafter the motor is energized to rotate the rotating mask 17, different areas of the cathode are allowed to emit thus effectively moving the beam of electrons and a circumferential Weld on the workpiece 15 may be performed.

For this type of annular cathode, it is possible to weld a workpiece 15 with the Weld seam in the vertical plane and avoid therewith the runoff of molten metal problem described before.

In FIGURE 2 a linear version of the cathode structure of FIGURE 1 is shown. The cathode 25 is again surrounded by a shield 26. The shield 26 is further provided with a slidable member 27 having multiple apertures 28, 29 and 30 therein. The member 27 is moved relative to the frontal emitting surface 31 of the cathode 25 to thereby produce sliding electron beams from the various apertures. Motion may be imparted to the slidable member by appropriate mechanical means such as the motor of FIGURE 1. The aperture 28 has a particular shape for similarly shaping the electron beam obtained therethrough.

It thus may be seen that a movable electron beam of this invention is obtained by moving a nondissipating mask relative to a contoured cathode and the electron beam may be rotated or moved in linear directions therewith.

It is to be understood that the invention is not limited to the specific embodiments herein illustrated and described but may be used in other ways without departure from its spirit as defined by the following claims.

Having thus described our invention, what we claim is:

1. A device for moving an electron beam formed primarily by secondary emission of electrons between a cathode structure and an anode ina gaseous environment having a predetermined pressure suitable for producing a glow discharge comprising:

means connected to the cathode structure and the anode for establishing a glow discharge and a cathode fall for the production of an electron beam from the cathode structure,

said cathode structure having an electron emitting surface where the electrons emitted from said cathode structure are obtained primarily from the collision thereupon of particles traveling toward said emitting surface from the glow discharge,

a nondissipative shield selectively spaced from and surrounding said cathode structure for suppressing the glow discharge between the shield and the cathode and preventing said particles from impinging upon the emitting surface.

said shield having a masking portion positioned in confronting relationship with said electron emitting surface and movable with respect to said emitting surface, the portion having at least one aperture for exposing a limited region of the emitting surface, through which aperture said particles are permitted to reach the exposed region to emit an electron beam, and I means for moving said portion relative to the emitting surface for producing relative motion of the electron beam.

2. A device as recited in claim 1 wherein said cathode structure is annular about a concentric axis and. is provided with an annular circumferential electron emitting surface, and

where said portion of the shield is annular and concentric with the annular emitting surface and is mounted for rotation with respect to the surface, and

where said moving means rotates said portion relative to the emitting surface for producing a rotating electron beam through said aperture.

3. A device as recited in claim 2 wherein said electron emitting surface is the internal surface of the annular cathode structure and emits the beam of electrons inwardly towards said axis, and

where said portion is interposed between said axis and said emitting surface.

4. A device as recited in claim 1 wherein the portion is provided with a plurality of apertures distributed for producing a plurality of electron beams.

References Cited UNITED STATES PATENTS 1,997,986 4/1935 Thomas 313-449 X 3,383,541 5/1968 Ferreira 313-207 JAMES W. LAWRENCE, Primary Examiner RAYMOND F. HOSSFELD, Assistant Examiner US. Cl. X.R 

